This invention relates generally to methods and apparatus in paper making machines and, more particularly, to methods and apparatus for separating a web from the former wire and for transferring the web to a transfer felt which also comprises a press felt.
The separation of the web from the former wire of a former section of a paper making machine and its transfer to the press section is a critical step in the paper making process since this separation and transfer significantly influences the reliability of the operation of the paper making machine. It is well known that in paper making machines which operate at low speeds, it is possible to use an open draw based upon the difference in speeds of travel of the web carrying former wire and the press felt in the press section.
However, in paper making machines which operate at relatively high speeds or which manufacture thin and consequently low strength paper, it is necessary to use a closed draw in the separation of the web from the former wire and its transfer to the press section. The present invention relates to a method and apparatus for effecting the separation of the web from the former wire and its transfer to the press section of the closed draw type.
Closed draw arrangements for separating the web from the former wire and transferring the same to the press section generally include a transfer fabric or felt belonging to the press section of the paper making machine which is guided into contact with the web carried by the former wire. The transfer felt is urged against the web carried by the former wire such as by a rotating roller whereupon the web adheres to the transfer felt, the latter then carrying the web to the press section.
Generally, there are two types of closed systems for accomplishing the transfer of the web from the former wire to the press section. More particularly, the simpler of these arrangements, commonly termed "lick-up transfer", utilizes a wet transfer felt which contacts the web to "lick-up+ the same, the web adhering to the surface of the transfer felt due to its wet nature. The other type of closed transfer system is generally termed a "vacuum pick-up" system wherein a vacuum ensures the adherence of the web to the transfer felt. A vacuum pick-up system is generally more desirable than a "lick-up" system since the former provides a greater choice in the selection of the quality of the transfer felt, among other things. In this connection and especially in applications where the transfer felt also functions as a press felt, certain requirements must be taken into account in the choice of the felt, namely, the web should securely adhere to the surface of the transfer felt at the point of separation from the former wire while at the same time the transfer felt must function in an efficient manner at a water-removing press roll nip. These requirements, however, are often contradictory in that in order for the web to securely adhere to the felt at the separation and to remain in adherence to the lower surface thereof over a span between the pick-up point and the first press roll nip, the felt must be relatively wet. However, as the moisture content of the felt is increased to facilitate adherence of the web to the felt, the dewatering capabilities, i.e., the absorbency, of the felt is correspondingly decreased therefore rendering the water capacity at the press nip relatively in inefficient. This is a distinct disadvantage in conventional vacuum pick-up arrangements.
Vacuum pick-up systems which utilize separate pick-up suction rolls are known and are widely used in modern paper making machines. For example, such a system is used in a Fourdrinier machine wherein the web is separated from the former wire at a point located on the run of the former wire between the chauffeur roll and the draw roll, the former wire sloping during such run at an angle of about 45.degree. to the horizontal. The particular point at which the web is separated from the former wire and is transferred to the transfer felt is determined by the particular design of the wire and press sections and their mutual location. After the web is separated from the former wire and becomes adhered to the pick-up felt, the web carrying pick-up felt laps the pick-up roll through a sector of about 45.degree.-90.degree. whereupon the web carrying pick-up felt moves onward to the press section.
Such vacuum pick-up systems which utilize separate pick-up suction rolls are disadvantageous in that under certain conditions such, for example, as high speed and inappropriate felts, the change of direction undergone by the web carried by the pick-up felts on the pick-up suction roll causes the web to loosen from the pick-up felt due to the centrifugal forces acting thereon. In order to prevent this separation, the pick-up roll is usually provided with a suction zone that extends beyond the actual zone wherein the web is separated from the former wire. Although this provision insures that the web will be maintained adhered to the felt, the extension of the suction zone requires a corresponding considerable increase in the capacity of the vacuum system for the suction roller. In other words, in such systems the suction requires a greater vacuum capacity than in arrangements where it is only required to separate the web from the wire and attach it to the felt.
Furthermore, conventional suction rolls presently in use in modern large high-speed paper making machines have high energy requirements due to the fact that in addition to handling air passing through the web at the suction roll, the vacuum system must also handle air which passes through the suction roll holes into the suction zone of the roll during each revolution of the suction roll. The quantity of such "hole air" may, in the case of large, high-speed machines, be twice or even three times greater than the quantity of air which passes through the web. In this connection, see applicant's U.S. Pat. No. 4,172,759.
Thus, another significant disadvantage of conventional pick-up systems is the high energy requirements necessitated by the large number of suction rolls utilized therein as well as the high costs of the rolls themselves. Further, such suction rolls are quite noisy in operation.
In order to at least partially alleviate some of the problems discussed above, a stationary transfer suction box has been utilized for separating the web from the former wire rather than a pick-up roll. In this connection, see U.S. Pat. Nos. 3,309,263, 3,441,476, 3,537,955 and 3,528,881.